The present invention generally relates to block masks and charged particle beam exposure methods and apparatuses which use such masks, and more particularly to a block mask having repeating basic unit patterns and to a charged particle beam exposure method and a charged particle beam exposure apparatus which use such a block mask to expose the unit patterns in one shot.
Conventionally, the photolithography technique was popularly used to form patterns on a semiconductor wafer. But because the integration density of integrated circuits have recently become high, the exposure technique used to form patterns on the semiconductor wafer is about to change from the photolithography technique to the charged particle beam exposure technique which uses a charged particle beam such as an electron beam.
There are various kinds of charged particle beam exposure techniques such as the variable rectangular exposure technique and the block exposure technique, depending on the shapes of the patterns that may be generated at one time. The variable rectangular exposure technique varies the size of the exposed pattern by varying the overlap of an aperture for use in variable rectangular exposure and the beam spot of the charged particle beam. According to the block exposure technique, the charged particle beam is transmitted through a mask having repeating basic unit patterns, and the unit patterns are exposed in one shot even for complicated patterns. Hence, the block exposure technique is particularly effective when exposing patterns which are fine but most of the area to be exposed consists of repeating basic patterns, such as the case of the patterns of a 256 Mbit dynamic random access memory (DRAM).
FIG. 1 shows an example of a conventional electron beam exposure apparatus employing the block exposure. The electron beam exposure apparatus shown in FIG. 1 generally includes an electron gun 101, an electron lens system L1a, a plate 102 with a rectangular opening, an electron lens system L1b, a beam shaping deflector 103, a first mask deflector MD1, a dynamic mask stigmator DS, a second mask deflector MD2, a dynamic mask focus coil DF, an electron lens system L2a, a mask stage 105 mounted with a block mask 104, an electron lens system L2b, a third mask deflector MD3, a blanking deflector 106, a fourth mask deflector MD4, a reduction electron lens system L3, a circular aperture 107, a projection electron lens system L4, a main deflector (electromagnetic deflector) 108, a sub deflector (electrostatic deflector) 109, a projection electron lens system L5, a wafer stage 111 mounted with a wafer 110, and a control system.
The control system includes a central processing unit (CPU) 121, a clock unit 122 which generates various kinds of clock signals including an exposure clock signal, a buffer memory 123, a control unit 124, a data correction unit 125, a mask memory 126, and a main deflector setting unit 127. The CPU 121 which controls the operation of the entire electron beam exposure apparatus, the clock unit 122, the mask memory 126 and the main deflector setting unit 127 are coupled via a bus 128.
For the sake of convenience, the data correction unit 125 and the main deflector setting unit 127 are shown in FIG. 1 as including the functions of a digital-to-analog converter and an amplifier. In addition, a laser interferometer which measures the position of the wafer stage 111 and a stage moving mechanism which moves the wafer state 111 are respectively known from U.S. Pat. No. 5,173,582 and No. 5,194,741, for example, and an illustration and description thereof will be omitted in this specification.
An electron beam emitted from the electron gun 10 passes through the rectangular hole of the plate 102, and is deflected by the first and second deflectors MD1 and MD2 before passing a desired pattern portion of the block mask 104. The cross sectional shape of the electron beam is shaped depending on the desired pattern portion, and the electron beam is swung back to an optical axis of the system by the converging functions of the electron lens systems L2a and L2b and the deflecting functions of the third and fourth deflectors MD3 and MD4. Thereafter, the cross section of the electron beam is reduced by the reduction electron lens system L3, and irradiated on the wafer 110 via the projection electron lens systems L4 and L5, thereby exposing the desired pattern on the wafer 110.
Exposure pattern data related to the patterns to be exposed on the wafer 110, block data related to mask patterns on the block mask 104 and the like are stored in the buffer memory 123. The exposure pattern data includes main deflection data to be supplied to the main deflector 108 and the like. In addition, data related to the relationships of the deflection data and the mask pattern positions which are measured in advance prior to the exposure, correction data for correcting the deflection data to be supplied to the dynamic mask stigmator DS and the dynamic mask focus coil DF and the like are stored in the mask memory 126.
The exposure pattern data received by the CPU 121 from a host unit (not show) or the like and stored in the buffer memory 123 includes a pattern data code PDC which indicates which mask pattern of the block mask 104 is to be used for the exposure. The control unit 124 uses this pattern data code PDC to read from the mask memory 126 the deflection data for deflecting the electron beam to the position of the mask pattern to be used. The control unit 124 supplies the read deflection data to the first through fourth deflectors MD1 through MD4 which are used for the pattern selection. In addition, the deflection data read from the mask memory 126 are also supplied to the data correction unit 125. The deflection data are read from the mask memory 126 based on an exposure clock signal which is generated by the clock unit 122.
On the other hand, the main deflector setting unit 127 reads from the buffer memory 123 the main deflection data of the main deflector 108 based on the clock signal from the clock unit 122, and supplies the read main deflection data to the main deflector 108. In addition, the deflection data of the sub deflector 109, the deflection data of the beam shaping deflector 103, and the deflection data of the blanking deflector 106 are decomposed into shot data in the control unit 124 depending on the data stored in the buffer memory 123. The shot data are supplied to the corresponding sub deflector 109, the beam shaping deflector 103 and the blanking deflector 106 via the data correction unit 125. In other words, the control unit 124 depending on the data stored in the buffer memory 123, the control unit 124 obtains and supplies to the data correction unit 125 the size of the electron beam for the case where the variable rectangular exposure is to be made and the deflection position of the electron beam on the block mask 104. The data correction unit 125 corrects each of the deflection data of the electron beam which are dependent on the patterns to be exposed and are supplied from the control unit 124, based on the correction data read from the mask memory 126. The deflection data of the beam shaping deflector 103 determine the variable rectangular size of the electron beam, and the deflection data of the blanking deflector 196 are set for each exposure shot.
FIGS. 2A and 2B show an example of the block mask 104 for a memory. As shown in FIG. 2A, the block mask 104 is made up of a substrate 104a which is made of a semiconductor such as silicon or a metal, and a plurality of deflection areas 104-1 through 104-12 provided on this substrate 104a. A plurality of mask patterns are formed in each of the deflection areas 104-1 through 104-12. In the electron beam exposure apparatus that employs the block exposure, the range of the mask patterns which are selectable by deflecting the electron beam about a position of the mask stage 105 is determined in advance, and each of the deflection areas 104-1 through 104-12 are a square range with a side of 5 mm, for example, and corresponding to the determined range of the selectable patterns. For example, when exposing a pattern by selecting the mask pattern within the deflection area 104-8, the mask stage 105 is moved so that the electron optical axis of the electron beam exposure apparatus approximately matches the center of the deflection area 104-8.
FIG. 2B shows the construction of the deflection area 104-8. In FIG. 2B, RDR denotes a row decoder region, CR denotes a cell region, and CDR denotes a column decoder region. For example, 48 block patterns can be arranged within this deflection area 104-8, and each block pattern is recognized by the pattern data code PDC. In other words, the pattern data code PDC is a mark for reading the contents of the mask memory 126 in correspondence with each of the mask patterns based on the exposure clock signal from the clock unit 122, where the exposure clock signal has a maximum frequency of 10 MHz, for example. As described above, the mask memory 126 stores the relationships of the mask pattern positions and the deflection data for deflecting the electron beam to each of the mask pattern positions, the correction data to be supplied to the dynamic mask stigmator DS and the dynamic mask focus coil DF and the like. Such data are stored in the mask memory 126 by adjusting the electron beam prior to the exposure and obtaining the deflection data, the correction data and the like with respect to the deflection areas that are to be used.
When changing the deflection area to be used, the mask stage 105 is moved so that the electron optical axis of the electron beam exposure apparatus matches the center of the newly selected deflection area. As the mask stage 105 is moved, a deflection calibration is made with respect to the newly selected deflection area, and it is also necessary to rewrite the data within the mask memory 126. Although it is not impossible to provide this calibration process within the exposure routine of the CPU 121, this approach is not realistic in that the adjustments related to the calibration take considerable time. Accordingly, in the conventional electron beam exposure apparatus employing the block exposure, there was a problem in that the number of block patterns which are selectable during the exposure of the patterns of one kind or one layer is limited to the number of block patterns within one deflection area on the block mask 104. In other words, in the case of the example described above, only 48 block patterns can be selected. As a result, even if apertures for use in the variable rectangular exposure are arranged within one deflection area, it is still only possible to select the 48 block patterns and the apertures for use in the variable rectangular exposure.
If the patterns are highly repetitive, such as the case of the patterns of the memory, it may be possible to expose the patterns using only the 48 block patterns and the apertures for use in the variable rectangular exposure, for example. However, if the patterns are not of a memory and include patterns which are random to a certain extent, 48 block patterns are not enough to expose all of the patterns, and in addition, it is impossible to form all of the repeating patterns within one deflection area of the block mask 104. In other words, there is a limit to the number of block patterns that may be formed within one deflection area of the block mask 104, and if all of the patterns to be exposed cannot be formed solely by the block patterns within one deflection area, the only way to form the remaining patterns is to use the apertures that are provided for the variable rectangular exposure. However, the frequent use of the apertures for the variable rectangular exposure reduces the exposure speed and accordingly increases the exposure time, and there was a problem in that the throughput of the electron beam exposure apparatus deteriorates.